Not applicable.
Not applicable.
1. Field of the Invention
The present invention generally relates to video processing systems. More particularly, the present invention relates to filtering pixel data in a video processing system. More particularly still, the invention relates to technique for decimating a digital video image.
2. Background of the Invention
The consumer electronics industry has experienced a dramatic explosion in product development over the last 20 years. This explosion has been fueled by consumer demand coupled with significant advances in semiconductor technology that have lead to lower cost semiconductor devices incorporating significantly more functionality than previously possible. For example, a hand-held calculator from 20 years ago provided the ability to perform rudimentary mathematical operations. Today, a hand-held device can provide much of the functionality of a desktop computer system.
The visual display of information to the user is of particular importance to the consumer electronics industry. The most notable examples of visual displays include televisions and personal computers. Other types of consumer electronics, including stereo receivers and hand-held computers, also include visual displays. FIG. 1, for example, shows a typical video screen 20 such as may be used in television or personal computer systems. An object 26 is shown on the screen which generally includes multiple lines of pixels. A typical television format includes 480 lines of pixels (LINE 1, LINE 2, . . . , LINE 480), with each line comprising 720 pixels. The size and spacing of the pixel lines in FIG. 1 have been exaggerated for clarity.
Each pixel in a line is represented by one or more data values. For example, each pixel can be represented in a xe2x80x9cRGBxe2x80x9d format which includes red, green, and blue color components. Alternatively, each pixel can be represented in a xe2x80x9cYUVxe2x80x9d or xe2x80x9cYCrCbxe2x80x9d format. In either the YUV or YCrCb formats, the xe2x80x9cYxe2x80x9d value represents luminance (xe2x80x9clumaxe2x80x9d) which determines the brightness of the pixel. The U and V values represent chrominance (xe2x80x9cchromaxe2x80x9d) components which determine color and are calculated as the difference between the luminance components and the red and blue color values; that is, U=Yxe2x88x92R and V=Yxe2x88x92B. The Cr and Cb values also represent chrominance and are scaled versions of the U and V chrominance values.
Video systems often represent images digitally. That is, each pixel comprising the image is represented by a digital YCrCb value, or a value in accordance with another format. To facilitate the transmission and storage of moving pictures, which generally involve voluminous amounts of pixel data, various encoding and compression techniques are used to process the data. One such technique is the Moving Pictures Experts Group (MPEG) standard commonly implemented in digital video disk (DVD) drives and other types of video equipment.
It is often desirable to display other information on the screen besides the video itself. For example, it may desirable to display textual information on one portion of the display and video on another portion. Additionally, it may desirable to show simultaneously two different video images on different portions of the display. For example, two different television stations could be viewed simultaneously, or a television signal could be viewed on one part of the screen while a DVD video signal is displayed on another part.
Video signals that have been digitally encoded are generally encoded for subsequent viewing in a full screen format. Thus, if a video signal is to be shown on a television screen which has 480 lines of pixels, the signal is encoded in the full 480 line format. After decoding, the signal is ready to be viewed in the full 480 line format. If other information is to be shown simultaneously on the screen, it is preferable to reduce (xe2x80x9cdecimatexe2x80x9d) the size of the image so that it can be shown with fewer lines and columns of pixels. Decimating an image makes room for other information on the screen. The present invention generally relates to a technique for vertically decimating a digital video image.
One decimation technique that has been suggested involves xe2x80x9cline droppingxe2x80x9d in which predetermined lines of pixel values are omitted (i.e., dropped) from the initial image. If it desired to reduce the vertical dimension of an image by a factor of two from 480 lines to 240 lines, line dropping would involve dropping all of the even numbered lines (lines 2, 4, 6, etc.) from the image and retain the odd numbered lines, or vice versa (dropping the odd lines and retaining the even lines). Although a fast technique, line dropping results in an inferior quality image because the resulting image is based on only half of the lines comprising the original image.
Thus, an improved decimation technique is needed that creates higher quality images than is possible with line dropping techniques. Such a technique preferably would in some way take into account every line of pixels from the original image in generating the decimated image. Despite the advantages such a system would offer, to date no such system is known to exist.
The deficiencies noted above are solved in large part by a video decoder that decimates an input image to produce a decimated output image. The video decoder uses approximately every line of pixels in the input image to compute the lines of pixels in the decimated image. In accordance with the preferred embodiment, the video decoder includes a vertical decimation filter that computes an average, and preferably a weighted average, of luminance (luma) values associated with pixels from each of four lines in the input image. The decimation filter preferably computes a weighted average of lumas from four adjacent lines of pixels from the input image which may represent a frame or a field of video data. The weighted average preferably uses coefficients that weight each luma in the calculation differently.
After calculating all of the luma values for a particular line of the decimated image, the line number associated with the first of the four adjacent lines is incremented by four (in a field-based system) to determine the initial line number for calculating the next line in the decimated image. The new initial line number thus represents the first line number of the set of four adjacent lines used to calculate the new line in the decimated image. This technique advantageously results in a high quality decimated image because each line from the initial image is used to compute the decimated image (i.e., no line is dropped without being used to compute the decimated image).
These and other advantages will become apparent once the following disclosure and accompanying drawings are read.